This invention relates to the production of aminophenols and their N-acylated amide derivatives which are ordinarily used as polymeric stabilizers or anti-oxidants. More particularly, this invention relates to the production of these anti-oxidant compounds under mild catalytic hydrogenation reaction conditions, where severe losses in product yield have been experienced in the past.
N-acylated aminophenols are known to be useful as anti-oxidants in many different compositions, such as rubbers, resins and other materials which are subject to the deleterious effects of oxidative aging.
These compounds are generally prepared by nitrosation or nitration of a phenol, followed by reduction of the nitroso-group or nitro-group to the corresponding amine, and subsequently N-acylating the amine.
Although it has been known for quite some time that the reduction step of the process may be accomplished by catalytic hydrogenation under pressure, the normal practice has continued to involve chemical reduction of the nitro or nitroso groups with, for example, sodium hydrosulfite or sodium bisulfite such as the method disclosed in Netherlands Patent Application 6,505,775 (Chemical Abstracts Vol. 64, No. 12601d, 1966), because of poor yields, long reaction times, and relatively severe operating conditions, including excessively high temperatures and pressures. Additionally, the catalytic hydrogenation requires a cumbersome isolation of the intermediate from its reaction medium prior to acylation.
For example, as recently as 1979, R. H. Kline disclosed in his U.S. Pat. No. 4,152,319, Example 2, a two-step process for preparing the anti-oxidant N-(3,5-di-tert-butyl-4 hydroxyphenyl) methacrylamide including catalytic hydrogenation. In the first step, 23.5 grams of the nitrosophenol precursor was catalytically hydrogenated to produce the aminophenol at 50 psi and 50.degree. C. in a reaction medium of absolute ethanol over a period of one hour. In the second step, after isolating the aminophenol solution, the acid chloride, methacryloyl chloride dissolved in benzene, was reacted to produce the N-acylated aminophenol. However, the yield of 10 grams of product was only 35% of theoretical.
Later, in 1980, Mr. J. A. Dale disclosed in his U.S. Pat. No. 4,205,151, improved yields from the catalytic hydrogenation of nitrophenols in tetrahydrofuran at relatively short reaction times (15-20 hours) but at quite severe pressures and temperatures of between 50.degree. and 150.degree. C., and from 100 psi to about 1500 psi and very low concentrations. However, the hydrogenation required about 20 hours at milder operating conditions such as 100 psi and 50.degree. C. While Dale disclosed conducting the catalytic hydrogenation in an aprotic solvent such as tetrahydrofuran, poor results were achieved. Although Dale does disclose the prospect of augmenting his reaction medium with certain non-polar organic substituents, such as toluene, benzene and hexane, he demonstrates that the reaction cannot proceed in those solvents alone. For example, hexane alone as a reaction medium presented difficulty and therefore the tetrahydrofuran was employed. Even then, however, as explained in Dale's Example 2, when the hydrogenation was conducted at 50 psi and presumably room temperature, despite conducting the reaction for four days, he could achieve only a 50% conversion. Thereafter, Mr. Dale discloses that by carrying out the hydrogenation reaction in tetrahydrofuran (eliminating the hexane), increasing the pressure to 1,000 psi, and raising the temperature to 85.degree. C., he could achieve a 100% yield, and that at 80.degree. C. over 15 hours, under pressure of at least 200 psi, he obtained 100% yield.
Accordingly, a method for manufacturing N-acylated aminophenols beginning with catalytic hydrogenation at milder operating conditions, shorter reaction times, and yet having commercially acceptable yields, would be a substantial advancement in the art.